Solid state lighting (light emitting diodes (LEDs) and/or lasers) have been commonplace for decades, where devices emitting in the red and infrared wavelength region being most prevalent across industries. As commercial applications have grown, however, so too has the need for LEDs and lasers capable of emitting across different wavelengths, including the primary wavelengths associated with monitors and displays, i.e., red, blue, and green. With such a growing need, researchers have developed various ways of designing LEDs and lasers to emit at different ranges in the visible spectrum, as well as infrared and ultraviolet. Green-emitting light sources have garnered much attention recently, because of their potential use across applications, such as home/industrial lighting, full-color mobile projectors, optical data storage, and medical and military applications. Traditionally, green emitting sources were formed of bulk semiconductor diode-pumped lasers. More recently, lasers emitting over green wavelengths have been demonstrated using an Indium Gallium Nitride (InGaN)/Gallium Nitride (GaN) multi-quantum well active region. In contrast to traditional visible lasers, direct semiconductor visible laser diodes are miniature, more efficient and cheaper.
Despite the growing need for specific wavelength sources, primarily green emitting sources, InGaN/GaN multi-quantum well LED and laser design and fabrication have proved to be difficult. The mostly commonly available III-nitride materials and devices are along the c-plane of wurtzite structure; and the crystalline asymmetry leads to strong spontaneous and piezoelectric polarization fields. Consequently, electron-hole overlap is significantly reduced. The large amount of indium in the InGaN alloy also leads to indium rich clusters and composition inhomogeneity, which is detrimental to the performance of LEDs and lasers.
At the same time, the availability of bulk GaN substrates is currently limited, therefore foreign substrates (SiC, sapphire, and Si) are widely used. The resulting large lattice mismatch and thermal expansion discrepancy lead to high density of dislocations (108-109 cm−2), which makes it challenging to realize high performance III-nitride LEDs and lasers. Therefore, visible lasers on sapphire substrates are still limited in ultraviolet (UV), violet and blue range of wavelength.
Queren, et al. (see, D. Queren, A. Avramescu, G. Bruderl, A. Breidenassel, M. Schillgalies, S. Lutgen, and U. Strauss, Appl. Phys. Lett. 94, 081119 (2009)) and Miyoshi et al. (see, T. Miyoshi, S. Masui, T. Okada, T. Yanamoto, T. Kozaki, S. Nagahama, and T. Mukai, Appl. Phys. Express 2, 062201 (2009)) have recently demonstrated quantum well green-emitting lasers on c-plane bulk GaN substrates. The threshold current densities, however, are quite large due to the reduced electron-hole overlap caused by the quantum confined Stark effect resulting from the large polarization fields associated with this design, especially for the longer wavelengths.
In order to tackle the above-mentioned polar-plane-related problems and enhance the device performance, one proposed approach is to utilize non-polar/semipolar GaN low dislocation density bulk substrates for which the polarization fields are much smaller, such as m-plane {1-100} and a-plane {11-20}. Several groups have demonstrated blue/green quantum well lasers on non-polar/semipolar substrates. These include, (i) K. Okamoto, J. Kashiwagi, T. Tanaka, and M. Kubota, Appl. Phys. Lett. 94, 071105 (2009); (ii) Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama, and T. Nakamura, Appl. Phys. Express 2, 082101 (2009); (iii) Y.-D. Lin, S. Yamamoto, C.-Y. Huang, C.-L. Hsiung, F. Wu, K. Fujito, H. Ohta, J. S. Speck, S. P. DenBaars, and S. Nakamura, Appl. Phys. Express 3, 082001 (2010); and (iv) M. Ueno, Y. Yoshizumi, Y. Enya, T. Kyono, M. Adachi, S. Takagi, S. Tokuyama, T. Sumitomo, K. Sumiyoshi, N. Saga, T. Ikegami, K. Katayama, and T. Nakamura, J. Cryst. Growth 315, 258 (2011). However, the lack of large size non-polar and semi-polar substrates, the lower stability of the growth window and poor indium incorporation during growth of InGaN/GaN quantum wells on these substrates has impeded laser development.
In addition to the epitaxy related problems, problems related to the formation of the laser facets also need to be addressed. The existence of the foreign substrates makes the laser facet cleaving almost impossible. For lasers grown on bulk GaN substrates, most conventional studies utilized cleaved sides coated with high reflective dielectric coating, but this process lacks reproducibility. Additionally, the cleaving approach does not create perfect facets on nonpolar/semipolar plane lasers. It is therefore desirable to realize mirrors having smooth surface with techniques of high precision and high reproducibility for all the crystal planes.
Consequently, a need still remains for development of high performance blue-green emitting lasers along c-plane direction. This invention aims to radically improve the visible laser performance with InGaN/GaN quantum dots incorporated into the laser active region and laser facets formed by FIB etching/polishing or cleaving.